Display systems, as a medium for delivering information, are closely related to everyday life. The mainstream display system now is the liquid crystal (LCD) display system. Besides, the active matrix organic light emitting diode (AMOLED) display system begins to emerge in field of small size display. Both the LCD display system and the OLED display system need to convert a digital signal representing image information to an analog signal for displayed on a panel. This requires a digital to analog converter (DAC). In a display system, the design of DAC is the core module in a column driving chip. In a display system, as illustrated in FIG. 1, DAC adopts a resistor string to generate voltages required by each gray scale. A pass transistor logic (PTL) is used to select the gray scale voltage corresponding to the digital signal input to DAC. If a display system has 256 levels of gray scale, then 256 resistors are needed to generate 256 different voltages.
In LCD and OLED display systems, an analog signal used to drive a display screen is a middle-high voltage signal. The analog signal of the middle-high voltage is selected from the voltages divided by the above mentioned resistor string of DAC, which causes a middle-high voltage is applied to the resistor string. As a result, it is required that related devices including the pass transistor logic (PTL) in DAC are middle-high voltage devices. However, middle-high voltage devices have higher power consumption and occupy more chip area.